High quality electronics equipment, such as for military applications, is often designed around modules. Each module consists of a metal heat sink plate, circuit boards laminated to opposite sides of the plate, and a connector at one end of the plate. Several of such modules or panels are installed in a rack by inserting projecting upper and lower portions of the heat sink plate into upper and lower groove-like tracks of the rack and sliding the module into place. As the module is slid into place, the multiple contacts of the module connector mate with corresponding contacts of a rack connector. Finally, clamps are operated to tightly clamp the projecting upper and lower ends of the plate against a side of the track which forms a cold plate that removes heat from the module.
The module and rack connectors have interfitting polarizing keys that allow only one of several modules to be fully inserted into a particular position in the rack. If the wrong module is inserted at a rack position, the equipment will not operate properly, and in fact components of the module may be damaged by subjecting them to excessive voltages. One approach to the use of polarizing keys, is to use identical keys, but to establish the polarization of each key by its rotational orientation. Such polarizing keys have traditionally had hexagonal or other polygonal shapes, with each key of a mating pair cut in half so that two interfitting keys form a hexagon. The keys are formed so there is a slight gap between them to allow for manufacturing tolerances. Such keys have long been used in rack-and-panel electronic circuit apparatus. However, this type of key is difficult to use for a connector designed for heat sink clamping.
Where two mating keys, that are each almost half of a hexagon, are used in heat sink clamping, the gap between the keys must be increased. This is because one key may be moved slightly closer to the other during clamping of the heat sink plate. Such movement of the plate during clamping is small, such as 0.010 inch, but is significant where small keys are used. Where the gap between keys is increased, this reduces the contact area between two nonmating polarizing keys. Such reduced contact area between mismated keys, could allow a module that was placed in the wrong rack position, to be accidentally fully inserted so the connectors on the module mate with those on the rack, resulting in misfunction and possible damage. A polarizing key design which maintained a clearance between keys to allow for key displacement during heat sink clamping, but which provided large interference between mismated keys, would be of considerable value.